Mode I crack propagation in hybrid unidirectional woven composite thin laminates

Thin composite laminates (< 2 mm) with standard ply thicknesses (> 100 µm) often exhibit unfavourable damage characteristics under impact loading, such as significant matrix deformations and delamination’s. Conversely, thin laminates composed of thin plies (< 100 µm) demonstrate quasi-brittle properties, leading to failure modes dominated by fibre breakage. Approaches like Hybrid Unidirectional Woven Composite Laminates (HUWCL) and Selective Ply Level Hybridization (SPLH) have been explored in thicker laminates to strike a balance between matrix and fibre failure. This experimental study investigates the Mode I delamination behaviour at hybrid unidirectional and woven interfaces within thin laminates. Specifically, non-spread tow 150  g/m2 unidirectional (UD) layers are hybridised with spread tow (ST) UD, spread tow fabric (STF), and plain weave (PW) thin plies, interleaved at the mid-plane to assess the impact of fibre architecture on Mode I fracture toughness and the corresponding bridging laws. The study also examines the effect of relative angle change of the interleaves at the crack plane. Eight laminate configurations are evaluated: four with co-linear architectures (0°//0°) and four with angled orientations (45°//0°). Results indicate that hybridisation generally enhances Mode I fracture toughness, with ST UD plies at the crack plane showing the most significant improvement of 189 % compared to the non-hybridised control. Relative angle change of the interleaf at the crack plane largely had a positive effect on Mode I fracture toughness. This research provides insight into delamination behaviour between different architectures and ply orientations in thin laminates, which is currently lacking in the literature.